As used herein "biogenetic nutrients containing nitrogen and phosphorus compounds" means those compounds useful for cultivated plants, i.e. fertilizers.
Due to industrialization, urbanization and intensification of agriculture, contamination of waters is an ever increasing global problem.
In general, methods of sewage treatment and purification which have been recently used do not result in complete purification since the majority of the dissolved and colloidal contaminants and a considerable part of suspended solid particles are not removed.
The limitations of money available for investment and steadily increasing energy costs do not permit development of technologies for a perfect purification.
Water considered as purified usually contains a considerable amount of dissolved material which consists generally of nitrogen and phosphorus compounds. These are the most important nutrients for plants, therefore their recovery for a direct application in agriculture improves the economics of sewage purification.
In addition, there are waste waters mainly of industrial origin where the substances dissolved therein consist predominately of heavy metal salts. In general, these are toxic for the biosphere, but their recovery by conventional processes is too expensive. Sewage-sludge is produced in the sewage treatment. At present, however, there is much controversy as to the advisability of utilization of such sewage-sludges and the conditions of such use.
Today it is well known that all organic substances in sewage-sludge are valuable humus forming materials, but opinions differ as to the value of sewage-sludge as a nutrient. The most significant reservations concerning the application of sewage-sludge in agriculture involve the heavy metal content and the hygiene of its use because of the infections sewage-sludges can cause. Although all over the world research programs are focused on the incorporation of heavy metals in plants and therethrough into the animal-human organism chain, the matter has not yet been resolved. Recently, in some countries provisional rules came into existence which prescribe the maximum acceptable loading of soils for each heavy metal.
The three main well known processes for the treatment of sewage contaminated with organic substances are as follows:
mechanical; PA0 biological; and PA0 physico-chemical. PA0 (a). partial biological treatment; PA0 (b). complete biological treatment; PA0 (c). complete biological treatment with nitrification; and PA0 (d). complete biological treatment with sludge-stabilization the so-called total oxidation-process.
The so-called mechanical separation-processes are important steps in water and sewage treatment technologies. About 30% to 40% purification can be attained when solid suspensions are separated mechanically by specific weight differences.
The aim of secondary purification or biological means is to remove colloids and dissolved organic substances. The removal of organic substances by microorganisms results in the formation of inorganic substances such as dissolved nitrogen and phosphorus salts.
As for sludge-loading, the following sewage treatment systems can be distinguished:
For the above systems, one of the main differences is the time of aeration which is 0.75-1.5 hours in the case of system (a); 3.00 hours in the case of system (b); 8.00 hours in the case of system (c); and 24.00 hours in the case of system (d).
The energy needed for aeration increases with time, whereas the efficiency of oxygen consumption decreases with time due to the progress of oxidation. Since aeration requires the most energy in sewage treatment plants, the extent of oxidation should thus be considered from economic aspects.
Physico-chemical purification processes are used if dispersed solids cannot be settled and mechanically separated as, e.g. colloid suspensions (particle size 0.01-1.0 .mu.m), or if the biological decomposition of the substances is too expensive or even impossible. Coagulation, precipitation and filtration are the main physico-chemical processes in connection with this invention. Specific costs of physico-chemical processes are relatively high owing, among others, to the expensive reagents required.
Physico-chemical processes are mostly used in the third treatment step in order to remove material remaining and inorganic nitrogen and phosphorus compounds which were partly formed during the previous biological treatment. In this way, the water discharged from the sewage treatment plant is of good quality, i.e. it can be reused either in industrial processes, or can simply be discarded without causing any eutrophization in surface waters.
In general, the third treatment step follows the complete biological purification and the accompanying nitrification. It increases, however, considerably both investment and operation costs.
The biological denitrification technologies involve nitrogen removal from the sewage after it has been oxidized previously to nitrate from ammonium with high energy comsumption by using an expensive and complicated process.
A usual way for denitrification is with ion-exchange technology using synthetic resin bases. Due to the special features of this process it is mainly used in drinking water purification.
Other processes, e.g. reversed osmosis, are even more expensive, accordingly, their practical use is rather limited.
Soluble inorganic phosphorus compounds, mostly ortho phosphates, are removed in the third treatment step by chemical precipitation using aluminum or iron compounds.
Iron and aluminum salts, lime, polyelectrolytes and combinations thereof are used for removing suspended materials. Such methods are dealt with in the "Water Treatment Handbook" of Degremont, 5th Edition Halsbed Press Book, Jon Wiley and Sons, New York 1979(I); in "Gewasserschutz, Wasser, Abwasser" Vol. 17. Aachen, 1975, and in "Anwendung von Fallungsverfahren zur Verbesserung der Leistungsfahigkeit biologischer Anlagen" Institut fur Bauingenieurwesen V. Technische Universitat, Munchen, 1978.
The above chemical treatments result in about 20-90% metal removal by formation of insoluble hydroxides. In the usual pH range between 6 and 8, the ammonium content remains unchanged.
According to Hungary Pat. No. 175,558, approximately 90% of the orthophosphate present can be removed by adding 200-300 mg aluminum sulfate to one liter of waste water, whereas the ammonium content remains unchanged.
A process is described in Hungary Pat. No. 180,613 where COD is decreased by 30-40% with charcoal treatment, but neither the amount of the suspended material, nor the ammonium content are changed. The removal of ammonium is usually carried out by nitrification followed by denitrification in order to avoid the undesired accumulation of NO.sub.3.sup.- in the effluent.
Summarizing, the above processes have the following disadvantages:
The nitrification can be performed by a total biological oxidation which requires great investment and operating costs. Similar difficulties arise with denitrification. Moreover, the denitrification is very sensitive to temperature and the concentration of oxygen absorbed in water.
Moreover, medium should be added to the dentrification-biomass which further increases the expenses and complicates the operation.
The reagents or additives used in waste water treatments exert rather specific effects. In addition, they are of relatively high price, e.g. charcoal, and may bring about corrosion, e.g. aluminum sulfate.
The main objects of the present invention are:
(a) As a result of the purification process, water should be produced which can be reused or which must not exert impermissible detrimental effects on surface or underground water; PA1 (b) Based on advanced research results in agronomy, biogenetic nutrients for agriculture should be produced by the purification process making it possible to utilize harmless sewage-sludge in agriculture. PA1 (a) carry out an intense purification of contaminated water, which includes removal and/or recovery of solid, colloidal and dissolved materials by precipitating the gross of suspension and of nitrogen, phosphorus and metal contaminants in fewer technological steps than in conventional processes. This reduces the costs of investment and operation; and PA1 (b) intensify simply existing sewage purification plants.
A further object lies in changing the role of sewage treatment plants, which are now restricted only to the removal of impurities, by making it possible to use by-products produced therein which have a positive economic impact.
In order to achieve the objects outlined above, it is necessary to
To achieve the above objects, this invention provides a process which offers solutions to meet the special demands and opens new economic aspects.